The first few weeks of Boot Camp are all Push-Ups all the time. The exercise is convenient and effective, because it requires no equipment, no set-up time and no complex coaching cues. Push-Ups are safe, simple and effective. No wonder Army drill sergeants use them to get recruits strong, fast.

Another great thing about Push-Ups? They're versatile. With just a few adjustments, they can work your entire upper body—and even build strength throughout your core and legs. Here are six Push-Up modifications recommended by Rick Scarpulla, head coach of powerlifting for the U.S. Military Academy at West Point.

Elbows Tight to the Body

Targets: TricepsScarpulla Says: Keep your hands, arms and shoulders in a straight line next to your body with your elbows pointed at 6 o'clock. You'll work your lats and triceps much more than you would if your elbows were splayed out at an angle away from the body.

Feet or Hands Elevated

Targets: ShouldersScarpulla Says: Some people try to hit their shoulders by opening their stance and pushing their elbows out, but this puts the shoulders in a compromising position. Instead, elevate your feet or hands on a bench to isolate the shoulders in a healthier way.

Hands Form a Diamond

Targets: PecsScarpulla Says: People often get this one wrong, believing they're supposed to put their nose inside the diamond. You actually want to place your hands under your sternum. Remember: To work your pecs, always keep your hands inside your shoulders.

Knee-to-Elbow (Spider-Man)

Targets: CoreScarpulla Says: Start in the standard push-up position. As you lower your body to the floor, lift one foot off the ground, swing your leg to the side and try to touch your knee to your elbow. Keep your body in a straight line; a sagging back is a sign of a weak core, and a sagging leg is a sign of weak hips.

Explosive Plyos from the Knees

Targets: EverythingScarpulla Says: Performing Plyo Push-Ups from your knees lets you focus on making them explosive. Keep your hips and lower back squeezed tight on each rep, and don't pull up with your back. At the bottom of the Push-Up, catch your body instead of letting it crash to the ground.